Data transmission technology has been implemented which uses different dataorganization structures for data streams. An organizational structure that is becoming more and more prevalent in conjunction with the increasingly widespread broadband ISDN (B-ISDN) is the Asynchronous Transfer Mode (ATM), in which the data cells of the data stream are structured in the ATM cell format (referred to in the following as: ATM data cells). The fundamentals of the Asynchronous Transfer Mode (ATM) are described, for example, in "Grundlagen der Vermittlungstechnik" Fundamentals of Switching Technology!, by G. Siegmund, Decker Publishers, 1992, pp. 377-389 and in the German Laid Open Document No. A1-40 12 850. The ATM data cells have a header used, inter alia, for addressing, identification, and testing, and a payload to which useful data can be allocated. Measuring and testing devices are known (e.g., from the catalog "The First Total Testing Solution for Broadband", No. 5091-7305E, 1993, of the firm HEWLETT PACKARD with the designation HP 75 000 or from the catalog "ATM Test System ME3505A", No. ME 3505A-1, 1990, of the firm ANRITSU CORP., TOKYO, JAPAN), which test data transmission and/or data processing systems, whose data streams are comprised exclusively of ATM data cells (referred to in the following as pure ATM data cell stream). These devices have an interface, via which the ATM data cells are read into an input stage of the measuring or testing device for further evaluation and, if indicated, can be output, as needed, together with measuring results via an output stage of the measuring device. It is customary for the input stage and the output stage to be linked with one another via a so-called "local bus" to exchange data.
From "ntz", Vol. 45, 1992, issue 2, p. 92, a generator-analyzer application for ATM data cells is known, whereby an interface module accesses a B-ISDN transmission line in order to analyze or generate a data stream of two ATM data cells. The interface module thereby separates call signals from the data signals. Subsequently, only the ATM cell-structured data signals are analyzed by an external device (e.g. a logic analyzer), while a further processing, evaluation and/or manipulation of the separated call signals and/or of control signals are not mentioned and apparently are not possible.
Already installed data transmission or data processing systems work with so-called PDH (plesiochronous digital hierarchy) transmission processes, which do not use any ATM data cells, but rather data streams. Besides the useful data, the data streams additionally contain frame-forming structural data. With respect to the term "plesiochronous", reference is made to "Pulstechnik" Pulse Technology!, E. Holzer/ H. Holzwarth, Springer Publishers, 1984, page 312. In this publication two signals are described as being "plesiochronous" with respect to one another when their clock pulse frequencies deviate only slightly from one another within specified limits. The fundamentals and bit rates of hierarchical processes known per se are described in CCITT Rec., vol. III, fascicle III.3, G.702. The structural data are allocated in each case to a section of the data stream consisting of a plurality of data and are distributed in accordance with a specified instruction as spaced apart data (or cyclically interleaved data) in the data stream.
An increasing need is expected for installed systems to be operated with a data stream containing data cells in a PDH (Plesiochronous Digital Hierarchy) frame. In such a data stream, the structural data are distributed on a byte by byte basis in the stream of the ATM data cells. The structural data (PDH frame information) interrupt and divide the ATM cells (for example within the payloads of the ATM data cells). Because the structural data contained in the data stream are spaced apart, a data stream of this type is not suited for direct testing by means of a device designed for pure ATM data streams. In accordance with CCITT Rec. G.7xx and G.7zz or ETSI DE/TM-3007, 2, 34 or 140 mbit/s are used as data transmission rates (data rates or bit rates) in the case of PDH-framed ATM data cell streams. On the other hand, a data rate of 155 mbit/s is recommended for the pure ATM data stream.
To be able to further process the structural data and the measuring results obtained from this data in an existing measuring device, which, as described at the outset, is designed for pure ATM data cells, it would be conceivable, in principle, to feed this data into the local bus of the measuring device. This would be comparatively expensive, however, since in the case of the measuring device, the bus structure would have to be altered accordingly, which can lead to conflicts with measuring modules contained in the measuring device.
The German Laid Open Document No. A1-41 21 863 discloses a process for operating a buffer memory, in the case of which plesiochronous digital signals read in on the input side are adapted to a data rate required on the output side, in that filler or stuffing (bit rate justification) bits are inserted, as needed, in place of data bits at defined positions of the emitted, non-cell-structured data stream. Details about the contents of the filler or stuffing bits are not included in the German Laid Open Document No. A1-41 29 863.